Ancestral Ghost was a once a day power exclusive to the Dunmer in Morrowind. The ghost was weak, but served as a distraction in the early parts of the game. That's why I like your idea of a leveled ghost, as you progress it becomes stronger.
If possible this could be accomplished rather simply I think. It could also make it cooler imo. Have each leveled ghost
appear different. Use the Ghost NPC look and just add weapons and armor to them. Keep it with a Dunmer feel though if at all possible. So far the closest thing to actual Dunmer looking armor is the falmer armor sans the helmet(shudders). Just use apparel to make the ghost look unique. Leave their heads bare so we can tell it is a mer under there. You do not have to make the armor appear on the ghosts in the same strength that they do in the game. By this I mean Ebony armored Dunmer ghost can be weaker than the Falmer armored one. The Falmer armored one harkens back to the old days of legendary Dunmer heroes or some such. You could have the list like this.
Remember Helmet-less for appearance's sake. However appearance is subjective the armor and weapon descriptions are merely for appearance sake and are what I think is appropriate. If someone thinks different speak up. I have not seen all the weapons and armor in this game yet. Not even close I think.
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Summon Ancestral Ghost-Regular every day Dunmer ghost in clothing with no spells and iron weapons including a bow. Presumably one of your less auspicious ancestors, that was a merchant or farmer. Level 1-10.
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Summon Ancestral Warrior- Dunmer in a regular armor set. Fur, Leather, Iron, Steel. Has Steel weapons or maybe Orsish. Weak level spells like flames, sparks, firebolt and lightning bolts. Presumably one of your tougher yet unknown ancestors. Level 11-20,
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Summon Ancestral Guardian- Dunmer in a set of Elven, Orcish or Glass armors. Likewise weapons. Stronger spells. Presumably an Ancestor that was accomplished and respected, yet forgotten throughout history. Level 21-30.
-Summon Ancestral Hero- In Ebony armor and weapons (lets reserve Daedric for the Dremora). Strong spells. An ancestor who did much and is fondly remembered as a source of great pride and prestige in your house, whether or not you know of your ancestral lineage or not. Level 31-40
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Summon Ancestral Myth/Legend- In Falmer Armor, not the helmet. Whatever armor you think is appropriate. Daedric Longsword. Powerful spells. Like master level Destruction and Illusion spells. An ancestor so powerful that they only answer the calls of those in their family with power to match their own. Little is known of them but their deeds in the early wars against the Dwemer and Nords in the far distant past guaranteed the success of many battles against their ancient foes. They are an ancient and powerful spirit capable of changing the course of any battle. As strong as Dremora Lords if not slightly more powerful. Level 41+.
At level 50 you gain the ability to summon and banish them as a permanently bound summon.
Something like that. It is rough but there it is. Of course equal attention would need to go towards the other races I think for balance lol. How anyone accomplishes that is their own business.